This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
2. The same after the solution of the shell in weak acid.
* Whence the group has also received the name of Polythalamia, i. e. many-chambered.
(18). On removing the delicate calcareous shell by the assistance of a weak acid, the body of the animal denuded of its covering (fig. 2, 2) is found to be entirely soft: that portion which is lodged in the first compartment of the shell is colourless and of a crystalline transparency; but in each of the succeeding segments there may be detected a granular mass of a brownish colour, and not unfrequently the minute silicious shells of Naviculae, Bacillariae, and other forms of Infusorial organisms, the remains of which may be traced nearly as far as the umbilicus of the spiral.
(19). The pseudopodia of the Foraminifera probably entangle and lay hold of the minute bodies which serve as food, consisting of Diato-macese, Desmidieae, the smaller forms of Confervaa, etc, and draw these by their contraction into the substance of the animal, within which they may be seen through the transparent shell. It is not by any means constantly that their indigestible residua are cast forth again, for they sometimes accumulate in such numbers as even to choke up a considerable part of the cavity. The living gelatinous substance is occasionally seen to extend itself around the exterior of the shell; and pseudopodia may then be put forth from this extension as well as from the ordinary outlets.
(20). The Foraminifera are evidently composite fabrics evolved by a process of continuous gemmation, each gemma remaining in connexion with the body by which it was put forth; and according to the plan on which this gemmation takes place will be the configuration of the shell. Thus, if a bud should be put forth from one of these creatures in the direction of the axis of its body, and a second shell should be formed around this bud in continuity with the first, and this process should be successionally repeated, a straight rod-like shell will be produced, having many chambers communicating with each other by the openings that originally constituted their mouths, the mouth of the last-formed chamber being the only aperture through which the gelatinous body, thus composed of a number of segments connected by pedicles or stolons of the same material, can receive a supply of food. The successive segments may be all of the same size, or nearly so, in which case the entire rod will approach the cylindrical form, or resemble a line of beads; but it often happens that each segment is somewhat larger than the preceding, so that the composite shell has a conical form, the apex of the cone being the original segment, and its base the one last produced.
If each of the successively formed segments instead of being developed exactly in the axis of its predecessor should be directed a little to one side, it is obvious that a curved instead of a straight rod will be the result; and this curve may be increased until it become a spiral. The character of this spiral will depend in a great degree upon the enlargement or non-enlargement of the successively formed chambers; for sometimes it opens out very rapidly, every whorl being considerably broader than that which it surrounds, in consequence of the great excess of the size of each segment over that of its predecessor (fig. 2, l); but more commonly there is little difference between the successive segments after the spiral has made two or three turns. In many genera the new segments are added in concentric rings, each surrounding its predecessors, so as to form flattened disks varying in size from that of a pin's head to that of a sixpence. When such disks are subjected to microscopic examination, they are seen to be composed of concentric circles of cells (fig. 3) which communicate with each other by means of lateral passages, and which in the living state arc each of them filled by the sarcode whereof the living portion of the animal consists. In this case there can be no reasonable doubt that the radial extensions of the outermost zone issue forth as pseudopodia from the marginal pores, and that they search for and draw in alimentary materials in the same manner as do those of other Foraminifera.
Fig. 3. Structure of the calcareous disk of an Orbitolite: - a, the central cell; 5, circumambient cell; c, c, concentric zones; d, d, annular passages of the outermost zone. The same parts are shown by a vertical section passing in a radial direction at e, e; and at f,f, following the course of one of the zones.
(21). Where the growth of the disk takes place with normal regularity, it is probable that a complete circular zone is added at once. When the sarcode body has increased beyond the capacity of its enveloping disk, it may be presumed that its pseudopodial extensions proceeding from the marginal pores coalesce, so as to form a complete annulus of sarcode round the margin of the outermost zone; and probably it is by a deposit of calcareous matter in the surface portion of this annulus that the new zone of shelly substance is formed, which constitutes the walls of the cells and passages occupied by the soft sarcode body. Thus we find this simple type of organization giving origin to fabrics of by no means microscopic dimensions, in which, however, there is no other differentiation of parts than that concerned in the formation of the shell, every segment and every stolon (with the exception of the two forming the nucleus or centre) being, so far as can be ascertained, a precise repetition of every other, and the segments of the nucleus differing from the rest in nothing but their form.
The equality of the endowments of each segment is shown by the fact (of which accident has frequently furnished proof), that a small portion of a disk entirely separated from the remainder will not only continue to live, but will so increase as to form a new disk, the loss of the nucleus not appearing to be of the slightest consequence from the time that active life is established in the outer zones.